Apparatus for thermal cracking and quenching

ABSTRACT

An apparatus is described for the thermal cracking of hydrocarbons which includes a vertically disposed external shell housing a heating and reaction tower in the lower portion, a quenching tower in the upper portion, and an intermediate transfer line. A bundle of heating tubes is disposed in the heating and reaction tower and a bundle of quenching tubes in the quenching tower. Hydrocarbon feed stock is introduced through nozzles located above the heating tubes in the heating and reaction tower and with or without a diluent from high velocity nozzles located below the heating tubes. The heating and reaction tower is completely filled with molten metal which is circulated upwardly therethrough by the feed stock and diluent so that it flows around the heating tubes, up and over a weir, down between the heating and reaction tower and the external shell, and into the bottom of the heating and reaction tower again. Diluent nozzles are provided through the external shell as a primary means of causing the molten metal to flow from the overflow weir into the lower portion of the heating and reaction tower. A quenching medium, which may be a granular solid such as fluidized sand or a high boiling oil, fills the quenching tower and is violently agitated therein by upwardly flowing reaction products and gases. Deposition of coke on the inner walls of the apparatus is prevented by the molten metal in the heating and reaction zone and by the oil or sand in the quenching tower and transfer line.

United States Patent 1 1 Ozawa et al.

I APPARATUS FOR THERMAL CRACKING AND QUENCHING [73] Assignee: Mitsui Shipbuilding and Engineering Co. Ltd., Tokyo, Japan [22] Filed: Sept. 11, 1972 [21] Appl. No.: 287,873

Related US. Application Data [62] Division of Ser. No. 55,403, July 16, 1970, Pat. No.

[30] Foreign Application Priority Data July 18, 1969 Japan 44-56987 [5 2] US. Cl. 23/284, 23/277 R, 23/283 [51] Int. Cl B01j 1/00, B01] 6/00 [58] Field of Search23/277 R, 284, 285, 283, 288 E, 23/262; 48/92; 165/106; 260/683 R; 208/48 FOREIGN PATENTS OR APPLICATIONS 233,399 3/1958 Australia 48/196 R Primary Examiner.loseph Scovronek Attorney, Agent, or Firm-Howson and Howson; William E. Hedges [57] ABSTRACT An apparatus is described for the thermal cracking of hydrocarbons which includes a vertically disposed external shell housing a heating and reaction tower in the lower portion, a quenching tower in the upper portion, and an intermediate transfer line. A bundle of heating tubes is disposed in the heating and reaction tower and a bundle of quenching tubes in the quenching tower. Hydrocarbon feed stock is introduced through nozzles located above the heating tubes in the heating. and reaction tower and with or without a diluent from high velocity nozzles located below the heating tubes. The heating and reaction tower is completely filled with molten metal which is circulated upwardly therethrough by the feed stock and diluent so that it flows around the heating tubes, up and over a weir, down between the heating and reaction tower and the external shell, and into the bottom of the heating and reaction tower again. Diluent nozzles are provided through the external shell as a primary means of causing the molten metal to flow from the overflow weir into the lower portion of the heating and reaction tower. A quenching medium, which may be a granular solid Such as fluidized sand or a high boiling oil, fills the quenching tower and is violently agitated therein by upwardly flowing reaction products and gases. Deposition of coke on the inner walls of the apparatus is prevented by the molten metal in the heating and reaction zone and by the oil or sand in the quenching tower and transfer line.

2 Claims, 11 Drawing Figure APPARATUS FOR THERMAL CRACKING AND QUENCHING This is a divisional of application Ser. No. 55,403, filed July 16, i970, now US. Pat. No. 3,718,708, issued Sept. 18, 1972. I

This invention relates to apparatus for thermal cracking of hydrocarbons into gaseous olefins, particularly, ethylene, propylene and other oleflns.

it has been well known in an apparatus for producing olefins by thermal cracking of hydrocarbons to use heat carrier recycle systems, and it has been also well known to employ tubular quenchers for quenching reaction products. An apparatus of the heat carrier recycle system type employs in most cases fine grains as the heat carrier and a fluidized bed reactor. However, this system must be provided with a regenerator and a recycle system in order to remove cracked coke deposited on the surfaces of fine grains during the reaction. Accordingly, the system has the disadvantages of requiring a complicated regenerator and the recycle system and that the apparatus is subject to being eroded. In a process comprising tubular quencher, cracked coke deposits on the tube surfaces greatly which results not only in reduction of the heat transfer efficiency but also in inhibition of operation in some cases; particularly, when using oils of heavier fraction as raw material, this process can not be employed. Moreover, the problem of conventional processes as to deposition of coke in the transfer line between the reaction zone and the quenching zone has not been well resolved.

The object of this invention is to provide a novel apparatus which may be simplified by means of molten metals used as heat carrier and which can exhaust all the cracked coke formed during the reaction out of the reaction zone with the stream of gaseous and vaporous reaction products. Another object of this invention is to prevent deposition of cracked coke in the transfer line between the reaction zone and the quenching zone and deposition of cracked coke on the tube walls of the quenching tube bundles by filling up and fluidizing solid grains or oils of high boiling point in the area from the surface of molten metals to the upper part of the quenching tube bundles, as well as to prevent deposition by wetting the said tube walls with entrained molten metals.

According to the present invention, a heating and reaction tower is filled with molten metals, in which dilution agent spouting nozzles are provided for force circulating the molten metals. Heating tube bundles and raw material spouting nozzles are submerged. Just above the heating and reaction tower, there is connected a quenching tower which is provided with quenching tube bundles which are filled with fluidized solid grains or oils of high boiling point from the sur face of the molten metals to the upper part of the quenching tube bundles. Spouting of dilution agents or raw oils forces the molten metals to circulate and agitate. The molten metal which is heated by the heating tube bundles to a predetermined temperature, comes into contact with the raw oils spouted into the upper or lower portion of the heating tube bundles, and contributes to the thermal-cracking reaction. The floating solid grains or oils of high boiling point on the surface of the molten metals are kept in fluidization by means of reaction products and diluent coming out of the heating and reaction tower to preventcleposition of cracked coke in a transfer line and the quenching tube bundles in a quenching tower.

The present invention will be explained by the accompanying drawing which illustrates an apparatus wherein a quenching tower filled with solid grains is directly connected with a heating and reaction tower of a type of forced circulating action. of convection current.

A heating and reaction tower 2 is filled with a single metallic element of low melting point such as lead, tin, zinc, bismuth, and cadmium or alloys mainly consisting of these elements in a molten state. Liquid or vaporous hydrocarbons are, preferably after pre-heating, charged to a middle part ofthe heating and reaction tower 2, that is an upper part of heating tube bundles 3 positioned in the heating and reaction tower, through a feedstock supplying nozzle 1 alone or together with diluents such as water vapour. The hydrocarbons supplied through the feedstock supplying nozzle 1 come in contact with molten metals heated at 700 l,l00C in the heating tube bundles 3 to contribute to the thermal cracking reaction for a residence time of less than 0.5 second, and then convert to reaction products mainly consisting of gaseous olefins, particularly ethylene, propylene or other olefins and aromatics. Metallic elements of low melting point. or alloys mainly consisting of these elements in a molten phase, that is molten met als are accelerated and forced to ascend by the action of spouting hydrocarbons from the feedstock supplying nozzle 1, by the spontaneous circulating action of convection current due to heating in the heating tube bundles 3, by the action of ejecting and spouting diluents, from a high velocity spouting nozzle 4 for dilution agents such as water vapour positioned in the lower part of the heating tube bundles 3, or by the action of ejecting and spouting diluents from a diluent spouting nozzle 5 for regulating the molten metals. After contributing to the reaction, the molten metal overflow over an overflowing weir 6 and flow downward along the inner wall of the heating and reaction tower 2. Molten metal, thus, is convectively circulated, and depending on this action the heat of reaction necessary for the endothermic cracking reaction is transferred. in other words, the molten metal serves as a heat carrier. The molten metal, temperature of which falls to 600 l,000C due to the reaction, flows through an opening 7 provided at the lower part of the overflow weir 6, and is heated up to 700 l,l00C in contact with the heating tube bundles 3. The heating tube bundles 3 are heated by flowing high-temperature fluids such as high-temperature flue gas in the tubes or directly by burning fuels by burners, for instance, a tunnel burner system. The diluent high velocity spouting nozzle 4 is positioned in the lower part of the heating tube bundles 3 for accelerating the molten metal to flow upward and make a uniform ascending current. The circulating rate of the molten metal can be controlled by adjusting the spouting rate of the diluents from the said nozzle. To the said nozzle, not only diluents, but also raw materials can be supplied. The diluent spouting nozzle 5 for flow regulation is positioned at the opening 7 in the lower part of the overflow weir 6 for stronger flow adjustment, and the circulating rate can be varied by adjusting the spouting rate fromjthe said nozzle.

The cracked coke formed during the thermal cracking reaction has a specific gravity less than that of the molten metal, so that it is readily separated from the molten metal and exhausted together with reaction products from the heating and reaction tower 2, without circulating together with the molten metal. The reaction products accompanied with diluents, cracked coke and entrained molten metal enter a quenching tower 9 through a transfer line 8 directly joined with the upper part of the heating and reaction tower 2. lnside the quenching tower 9 quenching tube bundles 10 are provided for quenching the reaction products. Solid grains such as sand are contained in an area from the surface of the molten metals to the upper part of the quenching tube bundles 10 and are kept in a fluidized state by reaction products and diluents.

The transfer line 8 is formed in a smaller way in cross section and separates the heating and reaction tower 2 from the quenching tower 9. In and above the transfer line 8 there is provided a uniformly distributing structure 11 built up with fireproof materials to reduce the cross section at area, and by the structure the reaction products and diluents coming from the heating and reaction tower 2 are uniformly distributed to keep the solid grains in a uniform fluidization state in the quenching tower 9. The cross section of the transfer line 8 is reduced in order to raise the flow rate of the reaction products and diluents not to allow the solid grains to fall down frequently from the upper part and absorb heat of the molten metals. The solid grains kept in the fluidization state in the transfer line 8 collide against the wall of the transfer line and remove cracked coke which is going to deposit on it. Moreover, since the molten metals accompanied with the reaction products and entraining to the quenching tower 9 flow down along the inner wall of the transfer line 8 to the heating and reaction tower 2 wetting the said wall, cracked coke can never deposit on the wall.

The average temperature of the solid grain layer which is kept in fluidized motion in the quenching tower 9 provided with the quenching tube bundles 10 is kept at 200 600C. The temperature of the reaction products and diluents such' as water vapour ascending from the heating and reaction tower 2 is 600 1,000C, and they are quenched to 200 600C in contact with the solid grain layer and the quenching tube bundles 10. The, coke formed in quenching is apt to deposit on the walls of the quenching tube bundles 10, but it is immediately stripped off and removed by frictional collision of the solid grains. The entrained molten metals coming from the heating and reaction tower 2 also promote the action of the stripping off and removing of deposits on the tube walls. More particularly, the molten metals collide against the tube walls of the quenching tube bundles 1 0 and wet them to prevent the coke from depositing directly on the tube walls.

The cracked coke adhering on the surface of solid grains, the cracked coke stripped off from tube walls and other places, and the cracked coke formed in reaction in the heating and reaction tower 2 are pulverized by friction and collision in the solid grain layer, exhausted through an exhaust tube 12, together with the reaction products and diluents, and treated by a wellknown method. As modification of the present invention, a transfer line 8 and a quenching tower 9 can be directly connected to a reaction tower 2 which is not provided with a diluent spouting nozzle for flow regulation 5, an overflow weir 6 and an opening 7, a so-called reaction tower of a forced agitating type. In such a process, raw materials of hydrocarbons are preferably supplied from a diluent high velocity spouting nozzle 4.

According to the present invention, furthermore, the quenching tower 9 can be filled with oils having high boiling point, instead of solid grains. When'using the oils having high boiling point, cracked coke is not exhausted through an exhaust tube 12 but contained in the oils having high boiling point. Accordingly, it should be noted that Conradson carbon in the oils must not exceed 40 percent byweight.

Solid grains or oils of high boiling point and molten metals are made up intermittently from a make-up nozzle 13. On starting up the operation solid grains or oils of high boiling point and solid metals are supplied from a supply nozzle 14. In shutting down the operation, solid grains or oils of high boiling point are discharged from a discharge nozzle 15 and molten metals are exhausted from an exhaust nozzle 16. The entrained molten metals gather together in the lower part of a quenching tower 9 during the operation and flow down along the inner wall of a transfer line 8. When the surface of a solid grain layer or a layer of oils having high boiling point raises during the operation, the solid grains or oils of high boiling point are discharged from a discharge nozzle 17 by a well-known method to keep the surface of the layer in the quenching tower 9 at predetermined level. v

As clearly seen from the above description, fluid molten metals are used as heat carrier in this invention, there is no need to employ such a regenerator for heat carrier grains as used in an conventional heat carrier grain circulation system. Heating of the molten metal heat carrier and thermal cracking of the hydrocarbons can be performed successively by circulating molten metal heat carrier or agitating it in a tower. As the result, the apparatus is very simplified, the investment cost is low, the site area is reduced and the operation becomes very easy. As cracked coke formed in thermal cracking reaction accompanies reaction products to be exhausted from the heating andreaction tower, the reaction system is never inhibited by coke. Since the quenching tower filled with fluid solid grains or oils of high boiling point from the surface of molten metals to the upper part of the quenching tube bundles is installed just above the heating and reaction tower, the problem of coke deposition in the transfer line between the heating and reaction tower and the quenching tower is resolved by continuous removal of cracked coke due to collision of solid grains or flow of oils of high boiling point, and also the deposition of coke is prevented by molten metals wetting the inner wall of the transfer line. Cracked coke which is going to deposit on the walls of quenching tube bundles in quenching is similarly continuously removed by collision of solid grains or flow of oils of high boiling point, and cracked coke hardly deposits on the walls as entraining solid metals also collide against the walls and wet them. Thus, there can be treated hydrocarbons having high boiling point and those containing much Conradson carbon.

We claim:

1. An apparatus for the thermal cracking of hydrocarbons which comprises, an external shell having a heating and reaction tower in the lower portion thereof and a quenching tower in the upper portion thereof connected by an intermediate transfer line of smaller cross section than said reaction and quenching towers,

a bundle of heating tubes in the heating and quenching tower and a bundle of cooling tubes'in the quenching tower, a plurality of high velocity nozzles located below the heating tubes for introduction of feed stock and/or diluent, a plurality of nozzles above the heating tubes for introduction of feed stock, said heating and reacspouting diluent through said inlet to cause the molten metal to flow therethrough, and means for removing cracked products and gases from said quenching tower, said quenching tower during use being partially filled by a quenching medium which is violently agitated by the upward flow of cracked product and gases, whereby the inner walls of the apparatus can be maintained substantially free of deposited coke by the agitated quenching medium and circulated molten metal.

2. An apparatus of claim 1 having a structure disposed above the transfer line for uniformly distributing the upwardly flowing products and gases in the quench- 

2. An apparatus of claim 1 having a structure disposed above the transfer line for uniformly distributing the upwardly flowing products and gases in the quenching tower. 